1. Field of the Invention
The present invention relates to surface acoustic wave devices in general and in particular to one embodiment of a surface acoustic wave filter that has first and second grating structures separated by first and second transducers such that a symmetric signal generated in the substrate by the first transducer is not received by the second asymmetrically constructed transducer but is reflected by asymmetric mode changing grating structures that reflect the signal asymmetrically such that it can be received by the second transducer. In a second embodiment, the invention relates to a compound filter of a given length that has a first filter section with first and second transducers separated by a first 3db grating structure, each of the first and second transducers being positioned a distance, X.lambda., from the first grating structure, a second filter section with third and fourth transducers separated by a second 3db grating structure, each of the third and fourth transducers being positioned a distance of (N/2+1/4) .lambda. from the second grating structure where N.gtoreq.1 and is an integer. The combined output of the two independent filter sections is equivalent to the output generated by a filter section of twice the length of the compound filter and has greatly reduced side lobes in the out-of-band filter response.
2. Description of Related Art
SAW transversal filters can be constructed with excellent passband and rejection characteristics. However, the device filter length can become very large if narrow passband rejection characteristics are required. In contrast, resonant filter structures can achieve very narrow passband with stop passband rejection characteristics in a much shorter physical length. However, the ultimate out-of-band rejection of these resonant filters is generally inferior to that which can be achieved with a transversal filter structure.
The poor out-of-band rejection of a resonant-type or coupled-resonator filter is the effect of the direct coupling between the transducers. Such direct coupling causes side lobes outside the stop-band of the gratings in a (sine X/X).sup.2 function. A single resonant filter comprises two spaced in-line transducers in a cavity formed between two reflective gratings. The grating reflective responses are relatively narrow band. However, the direct coupling response between the two transducers is relatively wide with many unwanted out-of-band responses.
With present transversal filters, the output response is a function of the transduction response of the transducers plus the reflective response of the grating electrodes thus leading to the unwanted out-of-band response.
Further, in a standard prior art transversal SAW filter, the filtering function is generally achieved by varying the transduction function of the transducers. This may be accomplished in a number of ways. Finally, in the prior art, to get a narrow bandwidth, the transducer length must be very long. If the transducer is short, a wide bandwidth is obtained. Thus, the more narrow the bandwidth desired, the more space the transducer must take on the substrate because its length must be increased.
It would be advantageous to have a transverse surface acoustic wave device in which the transducers do not acoustically couple to each other except through the gratings. In such case, the output of the transducer would be the product of the transduction function of the transducer times the reflection response of the gratings and since the grating out-of-band response would be very low or near zero, the net response of the filter out-of-band would be very low. The gratings can be weighted in any well-known fashion, such as that disclosed in commonly assigned copending patent application Ser. No. 08/152,179 filed Nov. 11, 1993, and incorporated herein by reference in its entirety, to get substantially rid of any sidebands that exist. Such a device would be very low loss and compact. As stated, in the prior art, to get a narrow bandwidth, the transducers must be made very long. If the transducers are made short, a wide bandwidth is obtained. If two transducers do not acoustically couple to each other except through the gratings, the transducers can be made very short and compact and the gratings can be used to change the reflection mode or function so that the reflected signal would couple to the output transducer.
In prior art devices such as that disclosed by David P. Morgan in the 1993 Ultrasonics Symposium entitled "Simplifications for Analysis of SAW Multipole Resonators," pp. 166-172, two in-line transducers are coupled to each other only through a grating structure that has two parallel tracks with a .lambda./4 longitudinal separation of adjacent parallel tracks. Such structure causes a mode conversion from a transverse symmetrical wave to a transverse asymmetric wave that is received by the output transducer. It would be advantageous if such structure could be improved to further decrease the magnitude of the side lobes that are on either side of the center frequency.
Further in U.S. Pat. No. 4,178,571, an acoustic wave resonator is disclosed having first and second reflector grating structures separated by spaced first and second transducers and wherein acoustically coupled cavities between the first and second transducers and their respective grating structures are split into staggered pairs such that the first pair of cavities has a length X.lambda. and the second pair of cavities has a length of X+.lambda./4. This allows mode conversion by suppressing the response of the output transducer to acoustic waves received directly from the input transducer and receiving the acoustic wave reflected by the grating structure.
In French Patent No. 2,682,833, a similar acoustic wave structure is disclosed as a filter in which first and second transducer structures are separated by a grating structure and wherein, again, acoustically coupled cavities between grating structures and each of the transducers are split into staggered pairs such that the first pair of cavities has a length of X.lambda. and the second pair of cavities has a length of X+.lambda./4. Mode conversion is again allowed for the reasons given earlier.
A similar construction is shown in PCT Application No. PCT/FR92/00827.